In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. Ink is then transferred to the surface of a material upon which the image is to be reproduced. In some instances, ink is first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful to prepare lithographic printing plates typically comprise an imageable layer applied over the hydrophilic surface of a substrate. The imageable layer includes one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the element is considered as positive-working. Conversely, if the non-imaged regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital imaging has become increasingly important in the printing industry. Imageable elements for the preparation of lithographic printing plates have been developed for use with infrared lasers. Thermally imageable, negative-working imageable elements are described for example in WO 2004/074930 (Baumann et al.) and WO 2007/090550 (Strehmel et al.).
Particulate materials have been incorporated into lithographic printing plate precursors for various reasons. For example, organic polymer particles have been incorporated into such elements for improved press developability as described in U.S. Pat. No. 6,352,811 (Patel et al.). Nanopastes of metallic particles are described for imageable elements in U.S. Pat. No. 7,217,502 (Ray et al.). Core-shell particles have been included in imaging layers so they coalesce upon imaging as described for example in EP 1,057,622 (Fukino et al.).
It is also known to use silica particles in various imageable elements to modify surface properties or to thicken the coated layers. For example, EP 1,096,313A1 (Hanabata) describes the use of silica particles modified with surface phenolic groups in UV-sensitive photoresists, which modified particles tend to agglomerate and provide thickening in the coating formulations. Such phenolic groups, however, tend to inhibit free radical photopolymerization.